To install the plugin, place it in ~/Library/Application Support/Aperture/Plug-Ins/Export. This folder tree may well be missing on your Mac - if so, just create them. Note: The Library folder in your home folder is hidden by default on Lion - you can get to it by holding down the Option/Alt key and choosing "Library" from the Finder's "Go" menu.

Thanks to Daniel Kennett, hopefully 500px will embrace this on there Apps page.